The present invention relates to a nonaqueous electrolyte secondary battery which utilizes a carbonaceous material as the anode active material and a lithium compound as the cathode active material.
With the recent remarkable advance in high-performance miniaturized electronic machines such as video camera and headphone stereo, there has arisen a strong demand for a secondary battery having a higher capacity as the power source of these electronic machines. Conventional secondary batteries are lead-acid secondary batteries and nickel-cadmium secondary batteries. Nowadays active development is going on in the nonaqueous electrolyte secondary battery with a high energy density which utilizes lithium metal or lithium compound as the anode active material.
There is the other type of nonaqueous electrolyte secondary battery which utilizes a carbonaceous material as the anode active material and lithium-cobalt oxide (LiCoO.sub.2) as the cathode active material. Owing to the doping and undoping of lithium, it prevents the growth of dendrite and the powdering of lithium. Therefore, it has a good cycle life performance and easily meets the requirements for high energy density and high capacity.
The nonaqueous electrolyte secondary battery as mentioned above will be described with reference to FIGS. 1 and 2.
The nonaqueous electrolyte secondary battery shown in FIGS. 1 and 2 utilizes a carbonaceous material (carbon) as the anode active material and lithium-cobalt oxide (LiCoO.sub.2) as the cathode active material. This battery is produced in the following manner.
First, the cathode 1 is prepared as follows: A slurry is made from 91 parts by weight of lithium-cobalt oxide (LiCoO.sub.2) as the cathode active material, 6 parts by weight of graphite as the conductor, 3 parts by weight of polyvinylidene fluoride as the binder, and 100 parts by weight of N-methylpyrrolidone as the solvent. This slurry is evenly applied to both sides of a 20-.mu.m thick aluminum foil as the cathode current collector 10. After drying, the coated foil is pressed using a roller press into a 180-.mu.m thick beltlike cathode 1. This beltlike cathode 1 consists of the cathode current collector 10 and the cathode active material 11a and 11b in layer form in approximately the same thickness on both sides of the current collector 10. The cathode in each secondary battery may contain 10.4 g of the cathode active material.
Secondly, the anode 2 is prepared as follows: A slurry is made from 90 parts by weight of pitch coke as the anode active material, 10 parts by weight of polyvinylidene fluoride as the binder, and 100 parts by weight of N-methylpyrrolidone as the solvent. This slurry is evenly applied to both sides of a 10-.mu.m thick beltlike copper foil as the anode current collector 12. After drying, the coated foil is pressed using a roller press into a 180-.mu.m thick beltlike anode 2. This beltlike anode 2 consists of the anode current collector 12 and the anode active material 13a and 13b in layer form in approximately the same thickness on both sides of the current collector 12. The anode in each secondary battery may contain 4.4 g of the anode active material.
Thirdly, the above-mentioned cathode 1 and anode 2 are wound as many times as required, with a separator interposed between them, to form a winding 14 as shown in FIG. 2.
FIG. 2 is a partly enlarged sectional view of the winding 14 used in the battery as shown in FIG. 1. The winding 14 is formed by winding many times a laminate around the hollow core 15, said laminate consisting of the beltlike cathode 1, the beltlike anode 2, and a pair of separators 3a and 3b which are 25-.mu.m thick microporous polypropylene film. The layers are arranged in the order of the anode 2, the separator 3a, the cathode 1, and the separator 3b.
Finally, the winding 14 prepared as mentioned above is encased in a battery case 5 as shown in FIG. 1. For current collection from the cathode 1 and anode 2, the cathode 1 is provided with the cathode lead 16 which is welded to the explosionproof valve 8. Similarly, the anode 2 is provided with the anode lead 17 which is welded to the battery case 5. The battery case 5 is filled with an electrolyte prepared by dissolving 1 mol/liter of LiPF.sub.6 in a 1:1 (by volume) mixture of propylene carbonate and 1,2-dimethoxyethane, so that the winding 14 is impregnated with the electrolyte. On the top and bottom of the winding are placed the insulating boards 4a and 4b in the battery case. On the explosionproof valve 8 is placed the closing lid 7, with their peripheries in close contact with each other. The periphery is sealed by crimping the edge of the battery case 5, with the gasket 6 interposed. In this way the battery case 5 is closed.
Thus there is obtained a cylindrical nonaqueous electrolyte secondary battery, 20.5 mm in outside diameter and 42 mm in height. Incidentally, the lid 7 has a vent which is not shown in the figure.
This secondary battery should be charged before use because the anode active material is not doped with lithium when it is completed.
The nonaqueous electrolyte secondary battery as mentioned above has a capacity of about 1,040 mAH when charged up to 4.1 V with a constant current of 200 mA and then discharged down to an end voltage of 2.75 V under a load of 7.5 .OMEGA..
It was found that the nonaqueous electrolyte secondary battery as mentioned above becomes poor in charge-discharge characteristics when it undergoes overdischarging. The nonaqueous electrolyte secondary battery of this type is usually examined for charge-discharge characteristics by the test which is conducted under the abovementioned charge-discharge conditions. However, the present inventors subjected several nonaqueous electrolyte secondary batteries to overdischarge until their discharge voltage reaches 0 V. This test was run intentionally assuming that the batteries might be used in an electronics machine which is anomalous or has no cut-off voltage established. Even though the circuit was opened when the discharge voltage reached 0 V, the open circuit voltage was not restored. The batteries which had undergone overdischarge extremely decreased in capacity, and some of the batteries tested could not be charged at all. The charge-discharge characteristics which the secondary battery exhibits after overdischarge down to 0 V are very important for the practical use of the secondary battery. Therefore, it is essential to take measures to prevent the deterioration of the charge-discharge characteristics.
There are proposed in Japanese Patent Laid-open Nos. 228573/1988 and 314778/1988 to incorporate of a compound (such as MoO.sub.3, V.sub.2 O.sub.5, and TiO.sub.2) into a cathode active material, such as MnO.sub.2. These patents tried to avoid disadvantage caused by the overdischarging, however, these teachings are different from the present invention which will be explained in detail rater.